This invention relates generally to optical encoders and more particularly to metal encoder disks.
An encoder is an electro-mechanical device used to convert the mechanical position of a shaft or axle to an analogue or digital electrical signal. The analogue or digital signal can then be used to determine position, direction, and velocity of the shaft or axle. Such encoders have been used in rotary switches or control knobs to detect movement of the switch and determine its position with a high degree of accuracy.
An encoder assembly of a transmissive type typically includes light emitters and detectors with an interposed rotating code disk. Light from the emitter (e.g. an LED) is directed toward the detector and interposed rotating code disk. The code disk includes patterned transparent portions and opaque portions arranged radially on the disk that act to alternately transmit and block light from the emitter from being detected by the photo-detectors as the disk rotates and its angular position changes. The encoder is programmed to index a detected position of the switch when certain designated photodetectors are blocked from the emitted light while others are activated. In disks with one or more tracks, each track consists of various combinations of opaque and transparent locations, arranged so that there is a unique combination of opaque and transparent locations along a radius at each possible angular position of code disk. That is, in an absolute position encoder, the photo-detectors will receive a unique pattern of light depending on the angular position of the code disk. Thus, the angular position of code disk can be determined according to which pattern of light is received at the photo-detectors.
In one type of incremental encoder 18 shown in FIG. 1, modules use a single-track index sensor 20 that sits inboard of a quadrature track 22. The index detector portion has two sets of photodiodes: those that drive the index ON (positive photodiodes, a minority of the surface area), and those that drive the index signal OFF (negative photodiodes, a majority of the surface area). The index pattern on the disk is constructed in such a way that at the index position all of the negative diodes are blocked and all of the positive diodes are illuminated, which causes the index to be asserted; at most other points on the disk's rotation, a plurality of negative diodes are illuminated 23 relative to the positive diodes, and the index is held inactive.
A typical code disk is constructed using a transparent plastic 24 with an opaque Mylar pattern 26 overlaid on the disk. On such disks, a ring of transmissive/clear material for the index track is allowed everywhere but where the index position is located; in that area there is a pseudo-random pattern of radial bars and windows that matches the arrangement of the positive and negative photodiodes of the sensor in the encoder module 20. In non-index positions of the disk rotation all of the elements, both negative and positive, are illuminated; because there are more negative photodiode elements, however, the index is kept in the OFF state.
Other types of code disks can be made from metal. Metal disk are desired for temperature compatibility or other reasons. However, metal disks are not naturally transparent so that if most of the index track is left open as a window (matching the window of the Mylar disk), then there is little to support the quadrature track which sits outside the radius of the index track.
FIG. 2 illustrates an examplary metal disk 30 known in the prior art with exterior quadrature track 32 and inboard index track 34. Disk 30 includes a radial index pattern 36 position within a small arc of the disk and a support pattern 38 outside of the arc formed of radially defined support structures and openings. The radial support pattern 38 may inadvertently trigger an index signal if positioned in such a way as to block the diodes of a certain type. This can occur because the orientation of the index pattern (and the underlying sensor pattern) and the support pattern is the same—that is radial—thereby potentially triggering potential false positive results. The presence of the radial support spokes means that it is unavoidable that a radial support spoke will at times cast a shadow on the pattern of the index sensor, and at certain positions will decrease the margin that keeps the index from triggering a false positive result. The other problem with the support spokes is the difficulty of proving that a given spacing and width of support spokes will be guaranteed to not produce false results, especially for complex multi-bit single-track index patterns.
Accordingly, the need arises for alternate designs that enable metal code disks in a transmissive-type encoder to maintaining structural rigidity without inadvertently triggering an index outside of the index points.